Stent having variable properties and method of its use

ABSTRACT

A stent and method of its use, the stent in its expanded configuration, exhibiting varying outward radial force along its length. In use, the expanded stent is of a tapered configuration which provides greater force in vessel regions requiring greater force and less force in regions requiring less. In particular the stent is useful in the ostium regions and at areas of bifurcation in vessels. Varying force over the length of the stent is achieved by varying the number of elements, the density of elements, the thickness of the elements making up the stent body, and maintaining a substantially metal to artery ratio in the expanded stent over its length.

CROSS REFERENCE TO CO-PENDING APPLICATION

[0001] This application is a Divisonal of U.S. application No.09/034,249 which is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

[0002] The invention relates generally to medical devices and their use.More specifically, the invention relates to stents for holding vesselssuch as arteries open to flow, particularly in the regions ofbifurcations.

BACKGROUND OF THE INVENTION

[0003] Stents are radially expandable endoprostheses which are typicallyintravascular implants capable of being implanted transluminally andenlarged radially after being introduced percutaneously. They have alsobeen implanted in urinary tracts and bile ducts. They are used toreinforce body vessels and to prevent restenosis following angioplastyin the vascular system. They may be self-expanding or expanded by aninternal radial force, such as when mounted on a balloon.

[0004] Stents are generally tubular devices for insertion into tubularvessel regions. Balloon expandable stents require mounting over aballoon, positioning, and inflation of the balloon to expand the stentradially outward. Self-expanding stents expand into place whenunconstrained, without requiring assistance from a balloon. Aself-expanding stent is biased so as to expand upon release from thedelivery catheter.

[0005] A vessel having a stenosis may be viewed as an inwardlyprotruding arcuate addition of hardened material to a cylindrical vesselwall, where the stenosed region presents a somewhat rigid body attachedalong, and to, the elastic wall. The stenosis presents resistance to anyexpansion of the vessel in the region bridged by the stenosis. Stenosesvary in composition, for example, in the degree of calcification, andtherefore vary in properties as well.

[0006] The arcuate geometry of any stenoses present a variation inresistance along the vessel axis to stent outward radial force.Specifically, stenosed vessel resistance is often greatest toward themiddle, lessening toward the ends, with a rapid decrease at the start ofhealthy vessel tissue.

[0007] In some instances, as in regions of bifurcation, stenoses arebelieved to be flow related phenomena, see Chapter 21 of the “Handbookof Bioengineering” (Richard Shaloh & Shu Chin, McGraw-Hill Book Company,1987) which discusses atherosclerosis at vascular bifurcations.

[0008] The left and right common carotid arteries are typical of suchvascular bifurcations. These arteries are the principal arteries of thehead and neck. Both of the common carotid arteries are quite similar anddivide at a carotid bifurcation or bulb into an external carotid arteryand an internal carotid artery. In the region of the carotid bulb andthe ostium of the internal carotid artery, stenoses present a particularproblem for carotid stenting due to the large tapering of the vesselinterior from the common carotid artery (both the left and the right) tothe internal carotid artery. The region of the carotid bifurcation orbulb happens to be where stenoses most often occur, particularly in theregion of the ostium to the internal carotid artery in both of thecarotid arteries. Self-expanding stents are generally preferred forcarotid stenting due to the anatomical location being subject toexternal compression.

[0009] A conventional self-expanding stent optimally has a lengthgreater than the length of the stenosed region to be kept open. Currentstents present a substantially uniform outward radial force and auniform resistance to compression along their length. Currently, stentsdo not vary these forces to match vessel geometries or resistances. Aconstant force stent, i.e., prior art stents, with sufficient force tomaintain an open channel within a stenosed vessel and to resistcompression, has greater force than necessary in the healthy vesselportion distal to the stenosis. The stent end may thus flare outward,protruding into, and possibly irritating non-stenosed tissue.

[0010] Stenoses can occur in vessel regions having asymmetric geometrylying on either side of the stenosis. One example of this is the ostiumof an internal carotid artery, having a wide opening converging into anarrower artery. A conventional stent placed in the region of the ostiumwould provide substantially uniform outward radial force over anon-uniform vessel diameter, that is, the force provided would begreater in a small diameter than in a larger diameter. If this force isproperly matched for the smaller vessel region, it is likely less thanoptimal for the larger region. Conversely, if this force is properlymatched for the larger vessel region, it is likely more than optimal forthe smaller vessel region.

[0011] What would be desirable, and has not heretofore been provided, isa tapered stent capable of providing sufficient force to keep a vesselopen within a rebounding stenosis, while providing only necessary forceagainst healthy, non-stenosed vessel regions. What else has not beenprovided is a tapered stent providing necessary, but only necessaryforce (outward force and compression resistance) along a stenosis in avessel region having non-uniform vessel diameter on either side of thestenosis. This is provided by the tapered stents of this invention whichexhibit differing radial force, cell size, geometry, flexibility andwhich provide substantially more constant metal to artery ratio (M/A)over their length. M/A is the ratio of the metal surface area of a stentto the surface area of the vessel or the like that the stent iscovering.

SUMMARY OF THE INVENTION

[0012] The present invention, in a preferred embodiment, includes aself-expanding stent of shape-memory metal having a tubular orcylindrical shaped structure in the unexpanded condition and a taperedtubular or cylindrical structure in the expanded or memorized condition,and in which the radial force varies longitudinally along the length ofthe stent. Also, its resistance to compression varies with length.Additionally, the cell design making up the stent is closed where forceand good plaque coverage and support is required and open whereflexibility is required. Additionally, the metal to artery ratio issubstantially more constant over the length of the stent when it isexpanded. One such stent is constructed of Nickel-Titanium alloy(nitinol). Other shape memory metals may be used. In one embodiment, thestent is constructed and arranged so that the outward radial force isgreater in the center and lesser at both ends. In another embodiment,the stent is constructed and arranged so that the outward radial forceis greater at one end and less at the opposite end. Such stents aresuitable for placement in stenosed and narrowing vessel regions such asthe carotid bifurcation and the ostial area associated therewith.

[0013] The stents of the invention may achieve a variation in radialforce along their length by including in the stent structural elementswhich intersect at connections having more metal in regions requiringmore radial force and less metal in regions requiring less radial force.The amount of intersection metal or strut member metal can be varied byvarying the size of the intersection area or the size of the struts.Greater or fewer connectors actually are used to vary the flexibilityalong the length of the stent more than increasing radial force. In apreferred embodiment, the stent structure is formed by laser cutting aNitinol tube, leaving a greater strut width and shorter length inregions requiring greater outward radial force and compressionresistance.

[0014] The struts of the invention are also characterized by the factthat they are constructed and arranged to present a substantially moreconstant metal to artery ratio over their length in the expandedcondition, i.e., expanded to a tapered shape.

[0015] The stent structure in a preferred embodiment includes a seriesof serpentine annular segments which are aligned to provide a tubularstructure. The segments are interconnected longitudinally. A desiredradial force can be varied by varying the stent strut dimensions in thisand other embodiments. In one embodiment, stent regions requiringgreater radial force have wider and shorter struts than regionsrequiring less force. The number of connectors between segments can alsobe varied for this purpose. It is also obtained by varying strut lengthand spacing and overall size. Another control is cell design per se.Closed cells provide greater plaque coverage and support than opencells. Closed cells are generally connected to cells in adjoiningsegments of the stent whereas open cells are not so connected. Thesefactors also provide control over properties such as flexibility andconformability. Cell geometry, i.e., closed and open, is used to providegood plaque support in the region of the stenoses (closed) and lesssupport (open) and more flexibility to either side of the stenoses.Also, closed cell structure may be used to bridge the origin of theexternal carotid artery or any other vessel side branch opening.

[0016] Generally speaking it is desirable to provide a stent of thisinvention with the aforementioned radial force which is variable overstent length in a predetermined arrangement; cell design which is closedin the area where the stent contacts plaque of a stenoses and more openwhere the stent contacts healthy vessel tissue; flexibility andconformability which is arranged to vary in a predetermined arrangementover the length of the stent, in both unexpanded and expanded condition.

[0017] Stents made in accordance with the present invention can providean outward radial force more closely matching the local forcerequirements in a tapered vessel. In particular, the stents providegreater force only where required at a stenosis, without providing toomuch force in the region of healthy tissue. The stents provide anexpanded geometry more closely tailored to the requirements of atapering vessel region. They are preferably stiff and strong at theproximal large diameter end or middle and weak and more flexible at thedistal smaller diameter end to provide strain relief and prevent kinkingof the vessel distal to the stent. The proximal end may also beflexible.

[0018] A stent of the invention with variable properties along itslength also applies to balloon expandable stents that can be used acrossbifurcations with large diameter change by dilating with a smallerballoon distally and a larger balloon proximally.

[0019] This invention is also concerned with a method for treatingstenoses in vessel bifurcation regions involving the use of a stent ofthe type described above.

BRIEF DESCRIPTION OF THE FIGURES

[0020]FIG. 1 is a schematic showing of a scenario 1 for carotidstenting;

[0021]FIG. 2a and 2 b are plots of force versus length of improvedstents for placement in FIGS. 1 and 7 respectively, i.e., an ostialstent and a bifurcation stent;

[0022]FIG. 3 is a schematic profile view of an expanded, tapered stentfor use in the scenario 1 of FIG. 1;

[0023]FIG. 4 is a flat plan view in detail of an unexpanded stent of thetype shown schematically in FIG. 3, including exemplary dimensions;

[0024]FIGS. 4a, 4 b, 4 c and 4 d are detail showings of portions of FIG.4;

[0025]FIG. 5 is an end view of the stent of FIG. 4;

[0026]FIG. 6 is a view showing the stent of FIG. 4 in the expandedcondition;

[0027]FIG. 7 is a schematic of a scenario 2 for carotid stenting;

[0028]FIG. 8 is a schematic profile view of an expanded, tapered stentfor use in the scenario 2 of FIG. 7;

[0029]FIG. 9 is a flat plan view in detail of an unexpanded stent of thetype shown schematically in FIG. 8, including exemplary dimension, and

[0030]FIG. 10 is an end view of the stent of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

[0031]FIG. 1 illustrates a narrowing vessel 52, such as the internalcarotid artery, having a wide region 56, a narrowed region 58, and astenosis (not shown) somewhere in between, i.e., in the cross-hatchedregion. The narrowing vessel of FIG. 1 illustrates the geometry as foundin an ostium at the bifurcation of the left common carotid 57, whereblood flows from the left common carotid artery 57 into the leftinternal carotid artery 59. The bifurcation also opens into the leftexternal carotid artery 60. An ordinary stent with sufficient force tohold open the wide region 56 would have greater force than necessary tohold open the narrowed region 58.

[0032]FIG. 2a illustrates a plot 66 a of outward radial force F along atapered, expanded stent length L for a stent embodying the presentinvention. The stent has a greater force in end region 68 a than at theopposite end region 70 a. A tapered stent having the force curve of FIG.2a is suitable for bridging a stenosis as illustrated in FIG. 1, havingsufficient force to hold open the wide region 56 of a vessel and lessforce in the narrow healthy tissue region 58 of the vessel, where lessis required.

[0033]FIG. 3 illustrates in schematic fashion a preferred nitinol stentembodiment of the invention producing a force distribution asillustrated in FIG. 2. Self-expanding stent 80 includes a conformabledistal end 82 for contacting healthy vessel tissue, and a stiffer,closed-cell proximal region 88 for providing increased plaque support.It has upon expansion a tapered diameter as shown. For example, a 0.236inch distal diameter and a 0.354 inch proximal diameter might betypical. These dimensions can be varied. Stent 80 is positioned on thedistal end of a delivery catheter, covered with a removable sheath,advanced to a stenosis to be crossed, and exposed for self-expansion byremoval of the sheath. Stent 80 expands radially to its memorizedtapered shape pushing against the stenosis and vessel wall.

[0034]FIG. 4 illustrates in more detail the nitinol unexpanded stentembodiment of FIG. 3 in flat plan view as a stent 100, having a middleregion 104 and end regions 106 and 108. Stent 100 has a tubular shape,shown in FIG. 5, formed of several serpentine segments 105, 107, 109,111 and 113, having a zig-zag pattern, each segment radially encirclinga portion of stent 100. Referring again to FIG. 4, segments 113 arelongitudinally interconnected by connectors 110, whereas the serpentinesegments 105, 107, 109 and 111 are all interconnected as shown in FIGS.4a and 4 b by direct connections 112. A preferred material forconstructing stent 100 is Nitinol. In this embodiment, the stent isformed by laser cutting a continuous-walled nitinol tube of diameter0.081 inches having a wall thickness of 0.006 inches, leaving only thestent structure as shown. Typical dimensions of various elements of thestent are shown in the Figure by way of example.

[0035] Referring now to FIG. 6, the stent of FIG. 4 is shown expandedand tapered. Since nitinol is a shape memory metal it can be formed intothe shape and size shown in FIG. 4, placed over a tapered tool andexpanded to a desired enlarged shape and size, such as the 0.236 inchdistal diameter and 0.354 inch proximal diameter previously mentioned,heated to a high temperature such as 500°C. to give it the memorizedsize and shape on the tool. The stent is then removed from the tool andcan be compressed for mounting on the delivery catheter.

[0036] By starting with a stent of nitinol having the dimensions setforth in FIG. 4, the expanded condition provides a stent having thedesirable properties described hereinbefore with reference to FIG. 3.All dimensions in the Figure are in inches. Of course, this is but oneexample of a stent according to the invention.

[0037]FIG. 7, similarly to FIG. 1, illustrates a narrowing vessel 52having a wide region 56, a narrowed region 58, a branching vessel 55 anda stenosis (not shown) somewhere in between regions 56 and 58, i.e., thecross hatched region. Again, narrowing vessel of FIG. 7 illustrates thegeometry as found at the bifurcation of the left common carotid artery57, where blood flows from the left common carotid artery 57 into theleft internal carotid artery 59.

[0038]FIG. 2b illustrates a plot 66 b of outward radial force F along atapered, expandable stent length L for a stent embodying the presentinvention. The stent has a greater force in its middle region 67 b thanat its end regions 68 b and 70 b. A tapered stent having the force curveof FIG. 2b is suitable for bridging a stenosis as illustrated in FIG. 7,having sufficient force to hold open the wide region at the ostium ofinternal carotid 59 and less force in healthy tissue at wide end 56 andnarrow end 58.

[0039] A stent for use in this cross hatched region will have propertiessuch as those to be described with reference to FIGS. 8 and 9, whichwill be different from the stent previously described with reference toFIGS. 1-6.

[0040] Referring now to the FIG. 8 schematic, stent 80 includes a middleregion 84 and end regions 86 and 87. The amount of radial force exertedper unit length of stent is greater in regions having shorter and widerstruts. As schematically illustrated in FIG. 8, stent 80 has shorter andwider struts in center region 84 than in end regions 86 and 87. Thus,stent 80 has a greater outward radial force and compression resistancein center region 84 than in end regions 86 and 87 making it particularlyuseful for stenting in the cross-hatched region of FIG. 7.

[0041]FIG. 9 illustrates in more detail the nitinol unexpanded stentembodiment of FIG. 8 in flat plan view as a stent 100 having a middleregion 104 and end regions 106 and 108. Stent 100 has a tubular shape,shown in FIG. 10, formed of several serpentine segments 105, 107, 109,111 and 113, having a zig-zag pattern, each segment radially encirclinga portion of stent 100. Segments 111 and 113 are respectivelylongitudinally interconnected by several connectors 110 whereasserpentine segments 105, 107 and 109 are all interconnected as shown indetail in FIGS. 9a and 9 b by direct connections 112. This embodiment isalso formed by laser cutting a continuous-walled nitinol tube ofdiameter 0.081 inches having a wall thickness of 0.006 inches, leavingonly the stent structure as shown. Typical dimensions of variouselements of the stent are shown in FIG. 9 by way of example.

[0042] Similarly to the stent embodiment of FIG. 4 as expanded to atapered shape shown in FIG. 6, the stent of FIG. 9 can be provided witha tapered memorized shape in the expanded condition. The stent willexhibit all of the desirable proportions heretofore described,particularly as discussed with reference to FIG. 2b. All dimensions inFIG. 9 are in inches.

[0043] The present invention provides a stent which when expanded to itstapered configuration, provides a radial force varied along stent lengthfor use in tapered anatomies. The stent has been described, in use, asbridging stenosed vessel regions for illustrative purposes. Another usein maintaining open channels through otherwise restricted body conduits.Stents used for other purposes are explicitly within the scope of theinvention.

[0044] It should be noted that although self-expanding stents have beenshown herein to illustrate the present invention, so called balloonexpandable stents can also include the variable radial force feature asdescribed herein. In the case of balloon expandable stents, however,these forces in general will be less than are necessary to expand thestent and thus the balloon will be used as known to those skilled in theart to complete the expansion of the stent. To obtain the tapered shape,two balloons of different diameter may be used to expand each end of thestent. These balloon expandable stents may be advantageously deployed inareas of a vessel such as at an ostium where a stent having more rigidor heavy members is desirable in the region of the stenosis, andenhanced flexibility in the distal portion of the stent is desired. Forexample, a balloon expandable stent can be made of stainless steel tothe design and dimensions shown in either FIG. 4 or FIG. 9. It should beunderstood therefore, that balloon expandable stents are also within thescope of the present invention.

[0045] In use, a stent of the self-expanding type, in unexpanded form,is placed on a delivery catheter and covered with a retractable sheath.The catheter is introduced into a vessel and advanced to a region ofbifurcation (ostium or bifurcation placement). The sheath is retracted,typically by pulling it in the proximal direction, to expose the stent.The stent then self-expands to contact the vessel wall and stenosis. Inthe case of a self-expanding stent such as the nitinol type describedherein, the stent expands to the tapered configuration upon beingexposed and exhibits the desired proportion described hereinbefore. Asheath is typically used for constraining a self-expanding stent. Aballoon expandable stent is typically crimped on to the balloon and notcovered by a sheath. In the case of a non-self-expanding stent, aballoon or other radial force means is inflated within the stent toexpand it. In the case of the stents described herein, two balloons maybe used sequentially to accomplish this. For example, a small balloonmay be used to expand the stent at the small diameter end of the taperedconfiguration. Then, a larger balloon may be used to expand the stentsat the large end of the tapered configuration. The catheter(s) arewithdrawn, leaving the stent implanted in the vessel. The method isadaptable depending on whether an ostial version or a bifurcationversion of the stent is being implanted.

[0046] Numerous characteristics and advantages of the invention coveredby this application have been set forth in the foregoing description. Itwill be understood, however, that this disclosure is, in many aspects,only illustrative. Changes may be made in details, particularly inmatters of shape, size, and arrangement of parts and in materialswithout exceeding the scope of the invention. The invention's scope is,of course, defined in the language in which the appended claims areexpressed.

What is claimed is as follows:
 1. A stent having an unexpandedconfiguration and an expanded configuration, the latter having a lengthand a varying diameter along said length, comprising in the expandedconfiguration: a tapered tubular shaped structure, said structure havinga radially outward biased force and compression resistance, said forcevarying along said length in a predetermined manner, said taperedtubular structure having a first end region of largest diameter, amiddle region of smaller diameter, and a second end region of smallestdiameter, wherein said force is weaker in said first end region,stronger in said middle region than in said first region, and weaker insaid second end region than in said middle region.
 2. The stent of claim1 including flexibility which varies along its length, the stent beingstiffest in the region of stenosis contact and more flexible at the endregions.
 3. The stent of claim 1 wherein the stent includes a relativelyclosed cell structure in the middle region compared to a more open cellstructure at the end regions.
 4. The stent of claim 1 wherein thestrongest region of the stent is at the first end of larger diameter. 5.The stent of claim 4 including flexibility which varies along itslength, the stent being stiffest in the region of stenosis contact andmore flexible at the end of small diameter.
 6. The stent of claim 4wherein the stent includes a relatively closed cell structure at thefirst end region of largest diameter as compared to a more open cellstructure at the other end region and the metal to artery ratio issubstantially constant over the length of the stent in the expandedconfiguration.
 7. The stent of claim 1 constructed and arranged as aself-expanding stent.
 8. The stent of claim 1 constructed and arrangedas a balloon expandable stent.
 9. The stent of claim 1 wherein the stentstructure is formed of a shape memory material so as to beself-expanding, the stent structure having an unexpanded configurationand an expanded memorized configuration, the unexpanded configurationhaving an average diameter less than that of the expanded configuration,the expanded configuration having a diameter which varies along thelength of the stent from a first end region, to a middle region, and toa second end region, wherein the second configuration diameter issmaller in said first end region, larger in said middle region than insaid first region, and largest in said second end region than in saidmiddle region or said first end region.
 10. The stent of claim 9comprised of nitinol.
 11. The stent of claim 9 wherein the stentincludes relatively closed cell structure in the middle region comparedto a more open cell structure at the end regions and the metal to arteryratio is substantially constant over the length of the stent in theexpanded configuration.
 12. The stent of claim 9 wherein the stentincludes a relatively closed cell structure at the first end region oflargest diameter as compared to a more open cell structure at the middleand other end region and the metal to artery ratio is substantiallyconstant over the length of the stent in the expanded configuration. 13.A method of stenting a bifurcation region or ostial region in a bloodvessel, comprising the steps of: providing an unexpanded stent havingpredetermined properties over its length in an expanded configurationincluding outward radial force and compression resistance; introducingthe stent onto a catheter; inserting the catheter and stent into animplantation region at bifurcation or ostium of a vessel; expanding thestent to an enlarged diameter of a tapered configuration in support ofthe vessel wherein the aforementioned properties are exhibited, andremoving the catheter leaving the stent implanted in the vessel.
 14. Themethod of claim 13 wherein the stent has a tapered tubular shapedstructure, said structure having a radially outward biased force andcompression resistance, said force varying along said length in apredetermined manner, said tapered tubular structure having a first endregion of largest diameter, a middle region of smaller diameter, and asecond end region of smallest diameter,.wherein said force is weaker insaid first end region, stronger in said middle region than in said firstregion, and weaker in said second end region than in said middle region.15. The method of claim 14 wherein the stent is implanted in thebifurcation region of the common carotid artery, extending across theopening into the external carotid artery and into the internal carotidartery, the end of largest diameter being in the common carotid arteryand the end of smallest diameter being in the internal carotid artery.16. The method of claim 14 wherein the stent is formed of a shape memorymaterial so as to be self-expanding, the stent structure having anunexpanded configuration and an expanded memorized configuration, theunexpanded configuration having an average diameter less than that ofthe expanded configuration, the expanded configuration having a diameterwhich varies along the length of the stent from a first end region, to amiddle region, and to a second end region, wherein the secondconfiguration diameter is smaller in said first end region, larger insaid middle region than in said first region, and largest in said secondend region than in said middle region or said first end region.
 17. Themethod of claim 16 wherein the stent is implanted in the internalcarotid with the end of largest diameter at the ostium thereof and theend of smallest diameter in healthy tissue downstream of the ostium.